The present invention, in some embodiments thereof, relates to holographic imaging and, more particularly, but not exclusively, to methods and systems of three dimensional measurements using holographic interferometry.
Holographic imaging, which records amplitude and phase information of light arriving from an object (such as integrated circuit (IC) semiconductor wafers or flat panel display (FPD)), may be used in microscopy to reconstruct the 3D profile of the object, i.e. the relative height of each point in the image.
Some methods use zero angle between the object and reference image, and a phase scanning mechanism, which, combined with multiple image acquisitions at the same object location, give the possibility to separate between the phase and amplitude information. These methods may even use illumination with a very short coherent length. However, such schemes are generally too slow for applications in which the object needs to be laterally scanned in limited time, such as wafer inspection.
In order to achieve fast lateral scanning of the object using holographic interferometry, it is desired to be able to extract the phase information from a single image. This may be done by introducing a non-zero angle between the object beam and the reference beam, and the use of coherent illumination. The spatial frequency in the image depends on the angle between the object imaging optical axis and the reference imaging optical axis. When the object has for example a raised surface, the interference lines shift. By analyzing the images, it is possible to extract the phase change of the interference lines, and from that deduce the height of the features in the object.
To be able to extract the phase information from the image, the interference lines need to be with a density low enough so that the camera pixelization will not average them out, but high enough to have a good lateral resolution of the phase information (this resolution is typically one cycle of interference lines).